The invention relates to a method and apparatus for interpreting thermal paint.
Thermal paint undergoes permanent colour changes when subjected to particular temperatures, with the final colour of the paint depending both on the temperature it is subjected to and the time period over which it is held at the raised temperature.
Thermal paint is commonly used in the development of high temperature components of turbines and combustors. The paint is applied to the component in a test situation, and subsequently analysed in order to determine the temperatures which different regions of the component reached during the test.
Conventionally, the analysis of the paint has been done manually. After heating, the colour profile of the paint is such that at certain temperatures the colour changes quite rapidly as perceived by the eye. At these points the temperature for the particular paint will have been calibrated empirically. Thus when the human operator finds the location of these perceived boundaries between colours, the temperatures at the boundaries can be determined and the temperatures between the boundaries interpolated.
The above process is time consuming for the human operator and, where the components include relatively inaccessible regions, can be very inconvenient. Further, the precise location where the colour change occurs may be subjective.
According to the invention there is provided a method for analysing a thermal paint applied to a part, the method including the steps of:
defining a colour space in at least two dimensions, the dimensions representing different colours and/or luminance values;
defining the location of calibration data comprising points, lines or curves in the colour space and retaining this information in the memory of a computer;
producing an image of the part to be analysed, the image comprising a plurality of pixels each containing colour information;
analysing the colour information to locate a pixel image point in colour space, for one or more of the pixels; and
using the computer to compare the pixel image points with the calibration data to determine the calibration point or location on the calibration line or curve which lies closest to the pixel image point in colour space.
Preferably the analysis of the colour information is carried out by the computer.
The colour space is preferably at least three dimensional, the three dimensions preferably representing red, green and blue light or a luminance value plus two out of three values representing the fraction of red, green and blue light present. However the colour space may include more than three dimensions and the dimensions may represent wavelength bands other than red, green and blue.
Preferably the calibration data represents colours in a standard set of colours. Preferably each colour in the set of colours represents a particular temperature reached by the thermal paint. Preferably the method further includes the step of determining the temperature represented by the calibration point or location on the calibration line or curve which lies closest to the pixel image point in colour space.
Preferably the calibration data comprises a plurality of points in colour space. Preferably the method includes the step of establishing the location of Voronoi polygons in colour space around the points in the calibration data. Preferably the method further includes the step of determining in which Voronoi polygon each pixel image point is located, in order to determine the calibration data point which lies closest to the pixel image point in colour space.
Preferably the calibration data points are divided into bands, each band containing a plurality of calibration data points. The method may include the step of checking in which band a particular calibration data point lies. The method may include the process of determining which of two calibration data points, each equally close to a pixel image point in colour space, is the correct calibration data point. This process preferably includes the step of checking in which band each of the two calibration data points lies and checking in which band the calibration data point assigned to one or more neighbouring pixel image points lie.
Preferably the part to be analysed is illuminated by a broad optical band width light source of substantially constant optical intensity across substantially the whole band width, which may be produced by a xenon flash. Preferably normally incident illumination is used.
The method may include the step of polarising the light using a polariser and filtering out directly reflected light using a cross-polariser.
Preferably the image is produced using a digital camera, preferably including three charge coupled devices.
The colour information may be filtered to remove unwanted information. A smoothing function may be used.
The colour information may be modified to compensate for brightness variations caused by the shape of the part or by camera distortions.
The colour information may be modified to compensate for non uniform illumination, or diffuse reflections.
Subsequently, the colour information may be analysed such that information relating to absolute brightness is substantially excluded.
According to the invention, there is also provided apparatus for carrying out a method according to any of the previous definitions.
According to the invention there is also provided apparatus for analysing a thermal paint applied to a part, the apparatus including:
means for defining a colour space in at least two dimensions, the dimensions representing different colour and/or luminance values;
means for defining the location of calibration data comprising points, lines or curves in the colour space and retaining this information in the memory of a computer;
means for producing an image of the part to be analysed, the image comprising a plurality of pixels each containing colour information;
means for analysing the colour information to locate a pixel image point in colour space, for each pixel; and
means for using the computer to compare each pixel image point with the calibration data to determine the calibration point or location on the calibration line or curve which lies closest to the pixel image point in colour space.
Preferably the apparatus includes a broad optical band width light source for producing light having a substantially constant optical intensity across the spectrum. The apparatus may include a xenon flash.
Means may be provided for polarising the light and filtering out from the image reflected light of generally cross-polarisation.
Preferably the means for producing an image of the part includes a digital camera, preferably including three charge coupled devices.
According to a further aspect of the invention there is also provided a method for analysing a thermal paint applied to a part, the method including the steps of:
defining a colour space in at least three dimensions, the dimensions representing different colours and/or luminance values;
defining the location of calibration data comprising points, lines or curves in the colour space and retaining this information in the memory of a computer;
producing an image of the part to be analysed, the image comprising a plurality of pixels each containing colour information;
analysing the colour information to locate a pixel image point in colour space, for one or more of the pixels; and
using the computer to compare the pixel image points with the calibration data to determine the calibration point or location on the calibration line or curve which lies closest to the pixel image point in colour space;
characterised in that the image pixels contain colour information represented by at least one more different colour and/or luminance values than are used to define the colour space.
According to a yet further aspect of the invention there is provided a method for analysing a thermal paint applied to a part, the method including the steps of;
defining a colour space in at least three dimensions, the dimensions representing different colours and/or luminance values;
defining the location of calibration data comprising part, lines or curves in the colour space and retaining this information in the memory of a computer;
producing an image of the part to be analysed, the image comprising a plurality of pixels each containing colour information;
analysing the colour information to locate a pixel image point in colour space, for one or more of the pixels; and
using the computer to compare the pixel image points with the calibration data to determine the calibration point or location line or curve which lies closest to the pixel image point in the colour space;
characterised in that the step of defining the location of calibration data also includes computing for each calibration point, line or curve a structure within the colour space which surrounds that calibration point, line or curve and with which all locations are closer to that calibration point, line or curve than to any other calibration point, line or curve in the calibration data.
Preferably the step of determining the calibration point or location on the calibration line or curve which lies closest to the pixel image point comprises determining within which structure the pixel image point lies.